One of the main goals of developmental biology is to elucidate the molecular mechanisms that govern the generation of distinct cell types from unspecified precursors. A detailed knowledge of these mechanisms will not only help us understand fundamental principles of normal ontogeny but also explain, and ultimately correct, instances where development has derailed and disease has resulted. The basic-helix-loop-helix-zipper transcription factor Mitf is encoded by a gene whose mutations in rodents or man may lead to abnormalities in the formation of neural crest-derived and neuroepithelial-derived melanocytes and to the concomitant skin pigmentation defects, deafness, and eye malformations that may lead to retinal degeneration. In mice, Mitf mutations appear to have different effects on neural crest-derived and neuroepithelium-derived melanocytes. Both in vivo and in culture, neural crest-derived mutant cells are blocked in their development at an early, pre-melanoblastic stage and may die. In contrast, mutant neuroepithelial-derived cells which normally become retinal pigment cells survive, display dysregulated expression of melanocyte-specific genes, and may finally develop into neuroretinal cells. Thus, Mitf appears to be a regulator of the developmental fate of at least two distinct cell lineages. To determine whether it has the capacity to respecify the fate of cells not normally destined to become melanocytes, we generated transgenic mice with ubiquitously inducible Mitf expression, are in the process of generating mice in which Mitf expression is specifically targeted to the developing retina, and are able to express Mitf ectopically in retinal and neural crest cell cultures. The analysis of target gene expression in such manipulated cells will help us understand the sequence of events that lead to melanocyte formation. To determine what is the precise fate of mutant neural crest-derived cells, we have initiated the generation of transgenic mice in which early melanoblasts and their derivatives are permanently marked.